Source follower contact

ABSTRACT

An image sensor includes a photodiode disposed in a first semiconductor material to absorb photons incident on the image sensor and generate image charge. A floating diffusion is disposed in the first semiconductor material and positioned to receive the image charge from the photodiode, and a transfer transistor is coupled between the photodiode and the floating diffusion to transfer the image charge out of the photodiode into floating diffusion in response to a transfer signal. A source follower transistor with a gate terminal is coupled to the floating diffusion to output an amplified signal of the image charge in the floating diffusion. The gate terminal includes a second semiconductor material in contact with the floating diffusion, and a gate oxide is partially disposed between the second semiconductor material and the first semiconductor material. The second semiconductor material extends beyond the lateral bounds of the floating diffusion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/728,893, filed on Oct. 10, 2017, which is hereby incorporated byreference.

TECHNICAL FIELD

This disclosure relates generally to semiconductor fabrication, and inparticular but not exclusively, relates to CMOS image sensors.

BACKGROUND INFORMATION

Image sensors have become ubiquitous. They are widely used in digitalstill cameras, cellular phones, security cameras, as well as, medical,automobile, and other applications. The technology used to manufactureimage sensors has continued to advance at a great pace. For example, thedemands of higher resolution and lower power consumption have encouragedthe further miniaturization and integration of these devices.

The typical image sensor operates as follows. Image light from anexternal scene is incident on the image sensor. The image sensorincludes a plurality of photosensitive elements such that eachphotosensitive element absorbs a portion of incident image light.Photosensitive elements included in the image sensor, such asphotodiodes, each generate image charge upon absorption of the imagelight. The amount of image charge generated is proportional to theintensity of the image light. The generated image charge may be used toproduce an image representing the external scene.

In CMOS image sensor processes a metal contact may be used to connectthe semiconductor wafer and transistor gates. The contacts betweenvarious pieces of device architecture are important because theelectrical properties of the contact (e.g., if the contact is Ohmic) maydictate device performance. Metal connections may be a source of darkcurrent.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive examples of the invention are describedwith reference to the following figures, wherein like reference numeralsrefer to like parts throughout the various views unless otherwisespecified.

FIG. 1 illustrates an image sensor pixel circuit, in accordance with theteachings of the present disclosure.

FIG. 2 illustrates a block diagram of one example of an imaging systemwhich may include the circuits of FIG. 1, in accordance with theteachings of the present disclosure.

FIG. 3 illustrates an image sensor pixel which may be included in theimaging system of FIG. 2, in accordance with the teachings of thepresent disclosure.

FIGS. 4A-4E illustrate a method of image sensor fabrication, inaccordance with the teachings of the present disclosure.

Corresponding reference characters indicate corresponding componentsthroughout the several views of the drawings. Skilled artisans willappreciate that elements in the figures are illustrated for simplicityand clarity and have not necessarily been drawn to scale. For example,the dimensions of some of the elements in the figures may be exaggeratedrelative to other elements to help to improve understanding of variousembodiments of the present invention. Also, common but well-understoodelements that are useful or necessary in a commercially feasibleembodiment are often not depicted in order to facilitate a lessobstructed view of these various embodiments of the present invention.

DETAILED DESCRIPTION

Examples of an apparatus and method relating to a source followercontact in an image sensor are described herein. In the followingdescription, numerous specific details are set forth to provide athorough understanding of the examples. One skilled in the relevant artwill recognize, however, that the techniques described herein can bepracticed without one or more of the specific details, or with othermethods, components, materials, etc. In other instances, well-knownstructures, materials, or operations are not shown or described indetail to avoid obscuring certain aspects.

Reference throughout this specification to “one example” or “oneembodiment” means that a particular feature, structure, orcharacteristic described in connection with the example is included inat least one example of the present invention. Thus, the appearances ofthe phrases “in one example” or “in one embodiment” in various placesthroughout this specification are not necessarily all referring to thesame example. Furthermore, the particular features, structures, orcharacteristics may be combined in any suitable manner in one or moreexamples.

FIG. 1 illustrates an image sensor pixel circuit 100, in accordance withthe teachings of the present disclosure. As illustrated, image sensorpixel circuit 100 includes first semiconductor material 101, photodiode103, floating diffusion 105, transfer transistor 107, source followertransistor 109, reset transistor 111, and row select transistor 113. Asshown, photodiode 103 is disposed in first semiconductor material 101,and coupled to transfer transistor 107. Floating diffusion 105 is alsocoupled to transfer transistor 107 to receive the image charge generatedin photodiode 103. Reset transistor 111 is coupled to floating diffusion105 to reset image charge in floating diffusion 105 after image sensorcircuit 100 has readout out the image charge. Source follower transistor109 is coupled to floating diffusion 105 to amplify image charge infloating diffusion 105, since the gate terminal of source follower 109is coupled to floating diffusion 105. As will be shown, a gate terminalof source follower transistor 109 may include a metal-free (e.g.,semiconductor) extension that contacts floating diffusion 105 directly,so there is only semiconductor material at the source followertransistor 109 floating diffusion 105 interface. A first terminal of rowselect transistor 113 is coupled to a second terminal of source followertransistor 109 to output image charge from image sensor pixel circuit100. As will be described in connection with FIG. 2, image sensorcircuit 100 may be included in a larger array of photodiodes, inaccordance with the teachings of the present disclosure.

FIG. 2 illustrates a block diagram of one example of an imaging system200 which may include the circuits of FIG. 1. Imaging system 200includes pixel array 205, control circuitry 221, readout circuitry 211,and function logic 215. In one example, pixel array 205 is atwo-dimensional (2D) array of photodiodes, or image sensor pixels (e.g.,pixels P1, P2 . . . , Pn). As illustrated, photodiodes are arranged intorows (e.g., rows R1 to Ry) and columns (e.g., column C1 to Cx) toacquire image data of a person, place, object, etc., which can then beused to render a 2D image of the person, place, object, etc. However,photodiodes do not have to be arranged into rows and columns and maytake other configurations.

In one example, after each image sensor photodiode/pixel in pixel array205 has acquired its image data or image charge, the image data isreadout by readout circuitry 211 and then transferred to function logic215. In various examples, readout circuitry 211 may includeamplification circuitry, analog-to-digital (ADC) conversion circuitry,or otherwise. Function logic 215 may simply store the image data or evenmanipulate the image data by applying post image effects (e.g.,autofocus, crop, rotate, remove red eye, adjust brightness, adjustcontrast, or otherwise). In one example, readout circuitry 211 mayreadout a row of image data at a time along readout column lines(illustrated) or may readout the image data using a variety of othertechniques (not illustrated), such as a serial readout or a fullparallel readout of all pixels simultaneously.

In one example, control circuitry 221 is coupled to pixel array 205 tocontrol operation of the plurality of photodiodes in pixel array 205.For example, control circuitry 221 may generate a shutter signal forcontrolling image acquisition. In the depicted example, the shuttersignal is a global shutter signal for simultaneously enabling all pixelswithin pixel array 205 to simultaneously capture their respective imagedata during a single acquisition window. In another example, imageacquisition is synchronized with lighting effects such as a flash.

In one example, imaging system 200 may be included in a digital camera,cell phone, laptop computer, automobile or the like. Additionally,imaging system 200 may be coupled to other pieces of hardware such as aprocessor (general purpose or otherwise), memory elements, output (USBport, wireless transmitter, HDMI port, etc.), lighting/flash, electricalinput (keyboard, touch display, track pad, mouse, microphone, etc.),and/or display. Other pieces of hardware may deliver instructions toimaging system 200, extract image data from imaging system 200, ormanipulate image data supplied by imaging system 200.

FIG. 3 illustrates an image sensor pixel 300 which may be included inthe imaging system of FIG. 2, in accordance with the teachings of thepresent disclosure. As shown image sensor pixel 300 includes photodiode303 disposed in first semiconductor material 301 to absorb photonsincident on image sensor 300 and generate image charge. Floatingdiffusion 305 is disposed in first semiconductor material 301 andpositioned to receive the image charge from photodiode 303. Transfertransistor (including—second—gate electrode 307) is coupled betweenphotodiode 303 and floating diffusion 305, both physically andelectrically, to transfer the image charge out of photodiode 303 intofloating diffusion 305 in response to a transfer signal (applied to gateterminal 307 of the transfer transistor). As shown a first lateral edgeof gate terminal 307 is vertically aligned with photodiode 303, and asecond lateral edge of gate terminal 307 vertically overlaps floatingdiffusion 305. Floating diffusion 305 includes dopant implanted intofirst semiconductor material 301 via ion implantation or the like, andthere is a portion that extends linearly into first semiconductormaterial 301 proximate to photodiode 403 and a rounded portion near thesource follower transistor.

Source follower transistor with (first) gate terminal 321 is coupled tofloating diffusion 305 to output an amplified signal of the image chargein floating diffusion 305. Gate terminal 321 of source followertransistor includes a second semiconductor material (distinct from firstsemiconductor material 301—although both first semiconductor material301 and the second semiconductor material may include the same materialcomposition, e.g., Si) in contact with floating diffusion 305. Theportion of second semiconductor material in contact with floatingdiffusion 305 may be a continuous piece of semiconductor extending fromfloating diffusion 305 to regions outside of the lateral bounds offloating diffusion 305. There is non-conducting gap between gateterminal 307 and gate terminal 321, and the thickness of the portion ofgate terminal 321 connected to floating diffusion 305 is less than thethickness of the rest of gate terminal 321. Gate oxide 325 is disposedbetween the second semiconductor material and first semiconductormaterial 301 where the second semiconductor material extends beyond thelateral bounds of floating diffusion 305. Image charge may flow fromfloating diffusion 305 into gate terminal 321 of the source followertransistor to amplify the signal from floating diffusion 305. However,the source follower transistor's gate terminal 321 is isolated fromother pieces of device architecture in first semiconductor material 301by virtue of gate oxide 325. Gate oxide 325 may include silicon oxide,halfnium oxide, or other metal and/or semiconductor oxides.

In the depicted example, image charge in floating diffusion 305 istransferred to gate terminal 321 of the source follower transistor tocreate the amplified signal (e.g., an electric field) in firstsemiconductor material 301. This is because at least part of an activeregion of the source follower transistor is disposed in firstsemiconductor material 301, and gate oxide 325 is disposed between theat least part of the active region and the second semiconductormaterial.

The architecture depicted—where there is no metal connection betweenfloating diffusion 305 and gate terminal 321 of the source followertransistor—presents meaningful advantages over other image sensorarchitectures that use metal to connect a source follower gate to thefloating diffusion, since the illustrated structure reduces dark currentthat may be caused by metal contacts. Moreover since, in the depictedexample, the image sensor operates at low speeds (relative to otherintegrated circuit applications) parasitic resistance matters less.Accordingly, the device architecture depicted conveys significantadvantages over other architectures using metal to connect the floatingdiffusion and source follower gate.

As illustrated, floating diffusion 305 is laterally disposed in firstsemiconductor material 301 between photodiode 303 and trench isolationstructure 323. Second semiconductor material extends over trenchisolation structure 323 (and is in contact with trench isolationstructure 323) so that trench isolation structure 323 is disposedbetween first semiconductor material 301 and the second semiconductormaterial (e.g., the semiconductor material included in gate terminal321). In one example, trench isolation structure 323 includes at leastone of a metal oxide, semiconductor oxide, semiconductor, or metal. Insome examples, a conductive core (e.g., metal or doped semiconductor)may be surrounded by an insulator (e.g., metal oxide or semiconductoroxide) to form trench isolation structure 323. The conductive core maybe charged to induce an opposite charge in first semiconductor material301 to further prevent unwanted charge transfer between pieces ofsemiconductor device architecture.

In the depicted example, the transfer transistor includes a gateterminal 307 to receive a transfer signal, and both gate terminal 307and gate terminal 321 include polycrystalline silicon. Both gateterminals 307 and 321 are in contact with gate oxide 325 which isdisposed on the frontside surface of first semiconductor material 301.As shown, both gate terminals 307 and 321 have a portion that has thesame thickness of polycrystalline silicon (which may include dopantatoms such as boron, phosphorus, arsenic, or the like). Image sensor 300also includes reset transistor (shown in FIG. 1) coupled to the floatingdiffusion to reset image charge in the floating diffusion in response toa reset signal.

FIGS. 4A-4E illustrate a method of image sensor 400 fabrication, inaccordance with the teachings of the present disclosure. One of ordinaryskill in the art having the benefit of the present disclosure willunderstand that the figures depicted may occur in any order and even inparallel. Additionally, figures can be added to, or removed from, themethod in accordance with the teachings of the present disclosure.

FIG. 4A shows providing first semiconductor material 401 includingphotodiode 403 and floating diffusion 405 disposed in firstsemiconductor material 401. FIG. 4A also shows gate oxide 425 disposedon a frontside (which, in subsequent figures, will have the majority ofpixel circuitry on it) of first semiconductor material 401. In thedepicted example, thin semiconductor material 431 is also disposed ongate oxide 425, so that gate oxide 425 is disposed between firstsemiconductor material 401 and thin semiconductor material 431. Thinsemiconductor material 431 may also include polycrystalline silicon orother semiconductor materials.

In one example, at least part of an active region of the source followertransistor may be formed in first semiconductor material 401 by ionimplantation or the like. For example, a doped region of firstsemiconductor material 401 may be used as the active region of sourcefollower transistor.

As shown, shallow trench isolation structure 423 may also be formed infirst semiconductor material 401 to electrically isolate the variouspieces of microelectronic circuitry in first semiconductor material 401.In the illustrated example, floating diffusion 405 is laterally disposedbetween trench isolation structure 423 and photodiode 403. Trenchisolation structure 423 may be formed by etching a trench in firstsemiconductor material 401, and backfilling the trench with at least oneof a metal oxide, semiconductor oxide, semiconductor, or metal.

FIG. 4B depicts depositing photoresist layer 435 on the surface of thinsemiconductor material 431, so that thin semiconductor material 431 isdisposed between gate oxide 425 and photoresist layer 435. As shown partof photoresist layer 435 is removed above floating diffusion 405. Inother words, a hole in photoresist layer 435 may be formed that issubstantially the same size as the first portion of floating diffusion405. Thus, a hole of the proper dimension can be etched through gateoxide 425 and thin semiconductor material 431 to provide an exposedelectrical contact to floating diffusion 405. Photoresist layer 435 mayeither be a negative or positive resist. For example, photoresist layer435 may be hardened with light, and unexposed photoresist above floatingdiffusion 405 washed away with solvent. Alternatively, the entirephotoresist layer may be deposited and hardened/cross-linked, then theportion above floating diffusion 405 is exposed to light to cleave thepolymer chains so it can be washed away with solvent or the like.

FIG. 4C depicts etching gate oxide 425 to expose a first portion offloating diffusion 405, and also removing photoresist layer 435. In someexamples, this may include etching through both the gate oxide 425 andthin semiconductor material 431 with either a wet or dry etch. Forexample, a dry etch may be used to remove thin semiconductor layer 431,then photoresist layer 435 may be removed. After photoresist layer 435is removed, a selective wet or dry etch may be used to remove the gateoxide. In this example, thin semiconductor material 431 acts as a maskfor the second chemically selective etch step.

FIG. 4D shows depositing second semiconductor material 441 on thefrontside of the device to contact the first portion of floatingdiffusion 405 that was exposed by the etching steps. FIG. 4D expresslyshows not forming a metal connection between floating diffusion 405 andgate terminal of the source follower transistor. In other words,floating diffusion 405 and the gate terminal of source followertransistor are directly connected by semiconductor. As shown, some ofgate oxide 425 is disposed between first semiconductor material 401 andsecond semiconductor material 441. More specifically, gate oxide 425separates first semiconductor material 401 and second semiconductormaterial 441 everywhere but on the exposed portion of floating diffusion405, and above trench isolation structure 423. In the depicted example,second semiconductor material 441 includes polycrystalline silicon, butin other examples may include other semiconductor materials such asgermanium, gallium or the like. Alternatively, second semiconductormaterial 441 may be single crystalline which could be achieved throughspecific deposition or annealing conditions. Second semiconductormaterial 441 may be doped in a variety of ways to enhance conductivity.

FIG. 4E depicts removing a second portion of the second semiconductormaterial (e.g., via a wet or dry etch) to form gate terminal 407 oftransfer transistor and gate terminal 421 of source follower transistor.Gate terminal 421 includes the second semiconductor material in contactwith floating diffusion 405, and part of second semiconductor materialmay be separated from an active region of source follower transistor bygate oxide 425.

In some examples, a reset transistor may be formed that is coupled tofloating diffusion 405 to reset image charge in floating diffusion 405in response to a reset signal. Similarly, a row select transistor may beformed with a first terminal coupled to a second terminal of the sourcefollower transistor.

The above description of illustrated examples of the invention,including what is described in the Abstract, is not intended to beexhaustive or to limit the invention to the precise forms disclosed.While specific examples of the invention are described herein forillustrative purposes, various modifications are possible within thescope of the invention, as those skilled in the relevant art willrecognize.

These modifications can be made to the invention in light of the abovedetailed description. The terms used in the following claims should notbe construed to limit the invention to the specific examples disclosedin the specification. Rather, the scope of the invention is to bedetermined entirely by the following claims, which are to be construedin accordance with established doctrines of claim interpretation.

What is claimed is:
 1. An image sensor, comprising: a photodiodedisposed in a first semiconductor material to absorb photons incident onthe image sensor and generate image charge; a floating diffusiondisposed in the first semiconductor material and positioned to receivethe image charge from the photodiode; a transfer transistor coupledbetween the photodiode and the floating diffusion to transfer the imagecharge out of the photodiode into floating diffusion in response to atransfer signal; and a source follower transistor with a gate terminalcoupled to the floating diffusion to output an amplified signal of theimage charge in the floating diffusion, wherein the gate terminalincludes a second semiconductor material in contact with the floatingdiffusion, and wherein a gate oxide is disposed between the secondsemiconductor material and the first semiconductor material where thesecond semiconductor material extends beyond the lateral bounds of thefloating diffusion.
 2. The image sensor of claim 1, wherein the imagecharge in the floating diffusion is transferred to the gate terminal ofthe source follower transistor to create the amplified signal in thefirst semiconductor material.
 3. The image sensor of claim 1, whereinthe floating diffusion is laterally disposed in the first semiconductormaterial between the photodiode and a trench isolation structure, andwherein the second semiconductor material that is part of the gateterminal extends over the trench isolation structure so that the trenchisolation structure is disposed between the first semiconductor materialand the second semiconductor material.
 4. The image sensor of claim 3,wherein the trench isolation structure includes at least one of a metaloxide, semiconductor oxide, semiconductor, or metal.
 5. The image sensorof claim 3, wherein the transfer transistor includes a second gateterminal to receive the transfer signal, and wherein the second gateterminal and the gate terminal include polycrystalline silicon, andwherein both the gate terminal and the second gate terminal are incontact with the gate oxide disposed on the surface of the firstsemiconductor material.
 6. The image sensor of claim 5, wherein thesecond gate terminal and at least part of the gate terminal includeapproximately a same thickness of polycrystalline silicon includingdopant atoms.
 7. The image sensor of claim 5, wherein at least part ofan active region of the source follower transistor is disposed in thefirst semiconductor material, and the gate oxide is disposed between theat least part of the active region and the second semiconductor materialof the gate terminal.
 8. The image sensor of claim 1, further comprisinga reset transistor coupled to the floating diffusion to reset imagecharge in the floating diffusion in response to a reset signal.
 9. Theimage sensor of claim 1, further comprising a row select transistor witha first terminal coupled to a second terminal of the source followertransistor.
 10. The image sensor of claim 1, wherein there is no metalconnection between the floating diffusion and the gate terminal of thesource follower transistor.